The present invention relates in general to controlling sound level automatically in a vehicle and more particularly concerns novel apparatus and techniques for automatically controlling the sound level in a vehicle relatively inexpensively and with relatively little apparatus by taking advantage of the discovery that the noise level in a vehicle generally increases with the vehicle speed.
It is known that a listener to sound reproduced in a vehicle requires more volume to maintain the same perceived sound level as the noise level in the vehicle increases because the noise masks the desired reproduced sound. Thus, the listener typically must manually increase and decrease the volume as the noise level in the vehicle changes. One prior art approach for attempting to overcome this problem involves locating a microphone in the vehicle, processing the signal transduced by the microphone to provide a signal related to the noise level in the vehicle and altering the volume in accordance with the signal representative of the noise level. This approach has a number of problems. It is costly. In order to avoid oscillation the system must measure noise outside the spectrum of the audio signals being reproduced. Thus noises that do not mask signals may cause the volume to increase.